1. Field of the Invention
The present invention relates to a shielded cable-connector assembly. More specifically, the present invention relates to an improvement in a shielded cable-connector assembly comprising a connector having a plug or a receptacle connected to the end portion of a multicore cable including a plurality of cores covered with a shielding member, for example, a shielding mesh wire or a shielding tape and adapted for use in connecting an interface of a personal computer, for example.
2. Description of the Prior Art
Generally a computer is connectable to a plurality of terminal units such as a line printer, a floppy disk unit and the like and usually an interface bus cable-connector assembly is used for connection of a computer and terminal units. Recently, such computer has been used in a variety of applications including the field of measuring instruments. In particular, recently a personal computer has been used more often at home for personal use. However, employment of a personal computer at home could cause electromagnetic wave interference to a radio receiver, television receiver and the like. The reason is that harmonic components of a logic signal or a clock signal of the frequency of 1 to 4 MHz used in such computer may leak out and enter into a television receiver and the like. For this purpose of preventing this, a shielding device is provided in such computer or terminal units and a cable used in an interface bus cable-connector assembly is also constructed so that the cores are shielded with a shielding member. However, since the end of such interface bus cable-connector assembly is connected to a connector without employment of any shielding measures to the connecting portion of the connector, a harmonic component of such as a logic signal could leak out from the connecting portion.
FIG. 1A is a front view of a conventional interface bus cable-connector assembly which constitutes the background of the present invention. FIG. 1B is a plan view, partially fragmentary, taken along the line 1B--1B in FIG. 1A. Referring to FIGS. 1A and 1B, a conventional multicore cable 1 comprises a multicore of say twenty four conductor cores 11, a shielding mesh wire 12 covering the same, and a jacketing cover 13 covering the shielding mesh wire 12. A connector 2 comprises a plurality of contact elements 21 mutually insulated from each other, a metallic contact cover 22 enclosing these contact elements 21, a fixing portion 23 formed integrally of the contact cover 22, and a connector cover 24 formed as front and rear half shells of the same configuration which are fixed together with screws. The connector cover 24 is formed with a cable inlet 25 at the end, and an engaging groove 26 is formed adjacent to the cable inlet 25. Two clamp members 19 made of metallic plates of the same configuration bent to have an approximately U-shaped configuration are fixed to the engaging grooves 26 with screws for preventing the multicore cable 1 from being detached from the connector 2. The multicore cable 1 is inserted from the cable inlet 25 and the end of the multicore cable 1 is fixed to the connector cover 24 by means of the clamp members 19. The respective conductor cores 11 of the multicore cable 1 are individually connected to the respective contact elements 21. The shielding mesh wire 12 is connected to one of a plurality of contact elements 21 by means of a drain line 14.
FIG. 2 is a diagrammatic view for explaining how to measure a shielding characteristic of an interface bus cable-connector assembly shown in FIGS. 1A and 1B. FIG. 3 is a graph showing a shielding characteristic of the interface bus cable-connector assembly shown in FIGS. 1A and 1B and of the embodiment of the present invention.
Now referring to FIG. 2, an inductive line L1 including 20 AWG soft copper conductors of 0.813 mm.0., for example, and an interface bus cable-connector assembly 1 shown in FIGS. 1A and 1B are disposed approximately 15 mm from a grounding copper plate, not shown, and in parallel with each other with a spacing of approximately 20 mm between the line L1 and the assembly 1. A signal of 0 to 200 MHz is supplied from a signal generator SG through a resistor R1 to one end of the inductive line L1. The other end of the inductive line L1 is connected through a resistor R2 of say 50 .OMEGA. to the ground. On the other hand, a voltmeter LM is connected through a resistor R3 of say 50 .OMEGA. to any of the contact elements 21 of the connector 2 coupled to one end of the bus cable assembly 1 and the contact elements 21 of the connector 2 at the other end are connected through a resistor R4 of say 50 .OMEGA. to the ground. The contact elements which are electrically connected to the shielding mesh wire 12 are connected at both ends to the ground.
When a signal of 0 to 200 MHz and of the voltage V1 is supplied from the signal generator SG to the inductive line L1 in the thus structured measuring circuit, any signal leaking from the inductive line L1 is prevented from entering into the multicore cable 1 inasmuch as the multicore cable 1 is shielded with the shielding mesh wire 12. However, no shielding countermeasure is provided at the connecting portion of the connector 2 and the multicore cable 1 shown in FIG. 1B and therefore a signal leakage from the inductive line L1 could enter into this portion of the assembly 1, whereby a voltage V2 could be displayed by the voltmeter LM. The shielding effectiveness can be evaluated by the following equation based on the above described voltages V1 and V2. EQU shielding effectiveness=20 log (V2/V1) (dB)
The shielding effectiveness for each frequency in the signal of 0 to 200 MHz is evaluated based on the above described equation. Then, as shown by the dotted line "a" in FIG. 3, a shielding effectiveness of merely 30 dB can be obtained in the vicinity of 120 MHz, for example. More specifically, when a harmonic component of a clock signal, a logic signal or the like in the vicinity of 120 MHz, for example, flows through the interface bus cable-connector assembly 1 shown in FIG. 1B, then the harmonic component leaks outside, whereby electromagnetic wave interference is caused in an FM radio receiver, a television receiver, or the like. A prior art interface bus cable-connector assembly of interest which solved such problems is disclosed in U.S. Pat. No. 3,744,128, entitled "Process for Making R.F. Shielded Cable Connector Assemblies and the Products Formed Thereby", and issued July 10, 1973 to the United States of America. The above referenced United States patent discloses an R.F. shielded cable connector assembly which comprises a multicore cable shielded by a shielding mesh wire, a connector housing, and a resin coating admixed with metallic flakes filled between the multicore cable shielded by the shielding mesh wire and the connector housing for fixing the respective cores and shielding the same. However, a resin material admixed with metallic flakes employed as a shielding material in the interface bus cable-connector assembly disclosed in the above referenced United States patent has large electrical resistance and hence the shielding effect thereof is 20 dB at the most, which is insufficient and has room for improvement. An attempt to increase the shielding effectiveness by increasing the amount of metallic flakes mixed into the resin in the above referenced United States patent can not improve the shielding effect so much in spite of an increase in cost. Since the interface bus cable-connector assembly of the above referenced United States patent involves contact through metallic flakes contained in the resin between the shielding mesh wire of the multicore cable and the connector housing rather than direct metallic contact between the shielding mesh wire of the multicore cable and the connector housing, stability of contact of the metallic flakes with the shielding mesh wire of the multicore cable and the connector housing is lacking. In particular, since the shielding material of the above referenced United States patent is formed by filling in of a resin material, deterioration of the shielding characteristic may be caused due to temperature changes during manufacture and application of a voltage thereacross during usage. In addition, a shielding material made of a resin material admixed with metallic flakes is inevitably weak in terms of mechanical strength and hence cracks may be caused in the resin material when connecting or installing the connector. Thus, a further disadvantage is seen in that this type of connector is not suited for an environment in which the connector must be often connected and disconnected.